PLOIDY AND DIVISION EFFICIENCY OF PETIOLAR PROTOPLASTS OF BRASSICA-NAPUS

The cellular heterogeneity in the DNA content of petioles of plantlets cultured in vitro was examined by flow cytometry after density gradie nt centrifugation. Three different subpopulations of petiolar protopla sts were separated using a mannitol/NaCl gradient. The first or upper fraction contained a substantial proportion of protoplasts with 4C nuc lei. The protoplasts had larger size, and displayed a higher division frequency. The third or lower fraction contained mostly 2C nuclei and had a lower cell division frequency, while the middle fraction with it s division frequency approaches that of the lower fraction. Approximat ely 50% of the mitotic activity of cells in culture derived from proto plasts of the upper fraction. Cells from petioles of different ages al so showed different extents of mixoploidy, with the older petioles hav ing higher ploidy levels. Within the same petiole, cells from the prox imal part contained higher ploidy levels than those from the distal pa rt.